In known methods and measuring instruments of the kind described above, the light emitters are operated with a constant luminous intensity and the light receivers generate basically sinusoidal analog signals, which are displaced in phase from each other and have a width corresponding to the pitch of the scale. In some of the conventional measuring methods, the outputs of a plurality of light receivers are combined to generate two quadrature signals (sine and cosine signals), which are evaluated. If a square wave signal is derived from the zero crossings of the two analog quadrature signals, four pulses will be obtained per scale increment. Frequency multiplier circuits may be used to derive more than two analog signals from the two quadrature signals and a square wave signal may be derived from the zero crossings of the analog signals. In that manner, up to 20 pulses can be generated per scale increment. But such circuits are costly and the waveforms of the signals may be distorted by interference and by a fluctuation of the d.c. component of the signals so that the electronic division of each scale increment may lead to objectively incorrect results.
It has also been proposed to generate pulses in a number which is theoretically as high as desired per scale instrument, usually pulses in a number of an order from 100 to 1000. Such methods and instruments are basically known from Laid-open German Application No. 29 42 080. As has been described, the light receivers generate two analog sine signals, which are displaced 90.degree. in phase and which contain a d.c. voltage component. A clock is connected by a frequency divider to a pulse-shaping circuit, which generates two sine a.c. voltages which are at a predetermined carrier frequency and are displaced 90.degree. in phase. The analog sine and cosine signals generated by the light receivers and the two sine-shaped carrier voltage signals which are displaced 90.degree. in phase are supplied to two frequency multipliers, which generate two a.c. voltage signals, which are at the carrier frequency and are displaced 90.degree. in phase and which have been amplitude-modulated by the analog sine and cosine signals generated by the light receivers. Said a.c. voltages are combined to produce a resultant signal, which is displaced in phase from a reference signal derived from the clock pulses by a second frequency divider. That resultant signal is supplied to a phase-controlled demodulator, to which the reference signal is delivered from said second frequency divider. The second frequency divider has the same frequency ratio as the first frequency divider.
The output voltage of the phase-controlled demodulator is delivered through a low-pass filter to a pulse generator, which supplies a counter with a signal that controls the counting sense of the counter. As a result, pulses in a number depending on the instantaneous position of the scanning unit, i.e., on the fraction of a scale increment which has just been scanned, are counted by the counter in a sense which depends on the direction of the scanning movement. Counting in a sense that depends on the direction of the scanning movement means that the pulses are counted in a count-increasing sense during a scanning movement in one direction and in a count-decreasing sense during a scanning movement in the other direction. The count of the counter may be indicated by a display.
Basic disadvantages are involved in the conventional operation of the measuring instrument with a constant luminous intensity, the generation of analog signals by the light receivers, and the direct conductive coupling of the resultant signals to the phase detector, which coupling results in errors if the d.c. voltages are different. The circuit arrangement which has been described involves also the disadvantage that two a.c. voltages in quadrature must be derived also from the clock pulses so that the signal-generating circuitry is further complicated. A particularly high expenditure is involved in the modulation of the a.c. carrier signals with the analog output signals generated by the light receivers by means of frequency multipliers, which are expensive and susceptible to interference. Besides, errors may be picked up from spurious signals. Errors may occur which cannot be detected and cannot be corrected. The known instrument comprises a single counter, which must be able to count at the clock frequency in continuous operation. The clock frequency must be high to ensure that errors will be avoided even when the scanning unit is moved at a high velocity relative to the scale. Pulses which are lost or picked-up interference may result in wrong counts, which are not corrected and the errors are cumulative during a measuring operation.